Aphid transmission studies using helper component proteins of potato virus Y expressed from a vector derived from potato virus X

Insect allies-Assessment of a viral approach to plant genome editing

The Insect Allies program of the Defense Advanced Research Projects Agency has already sparked scientific debate concerning technology assessment-related issues, among which the most prevalent is that of dual use. Apart from the issues concerning peaceful applications, the technology also provides the blueprint for a potential bioweapon. However, the combination of a virus-induced genetic modification of crop plants in the field using genetically modified insect vectors poses a greater risk than the hitherto existing use of genetically modified organisms. The technology's great depth of intervention allows a number of sources for hazard and a tendency towards high exposure, but it is also encumbered with notable deficits in knowledge. These issues call for a thorough technology assessment. This article aims to provide an initial characterization from a technology assessment perspective, focusing on potential sources of risk for this novel invasive environmental biotechnology at an early stage of research and development. Integr Environ Assess Manag 2022;18:1488-1499. © 2022 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

The HCPro from the Potyviridae family: an enviable multitasking Helper Component that every virus would like to have

RNA viruses have very compact genomes and so provide a unique opportunity to study how evolution works to optimize the use of very limited genomic information. A widespread viral strategy to solve this issue concerning the coding space relies on the expression of proteins with multiple functions. Members of the family Potyviridae, the most abundant group of RNA viruses in plants, offer several attractive examples of viral factors which play roles in diverse infection-related pathways. The Helper Component Proteinase (HCPro) is an essential and well-characterized multitasking protein for which at least three independent functions have been described: (i) viral plant-to-plant transmission; (ii) polyprotein maturation; and (iii) RNA silencing suppression. Moreover, multitudes of host factors have been found to interact with HCPro. Intriguingly, most of these partners have not been ascribed to any of the HCPro roles during the infectious cycle, supporting the idea that this protein might play even more roles than those already established. In this comprehensive review, we attempt to summarize our current knowledge about HCPro and its already attributed and putative novel roles, and to discuss the similarities and differences regarding this factor in members of this important viral family.

Full-genome analyses of a Potato Virus Y (PVY) isolate infecting pepper (Capsicum annuum L.) in the Republic of South Africa

Potato Virus Y (PVY) is a pathogen of economic importance in pepper and other major crop species in the family Solanaceae. Three major PVY strain groups: O, C, and N, have been distinguished on the basis of genome sequencing. In this study, the first full-genome sequence of a PVY isolate (JVW-186) infecting pepper from the province of KwaZulu-Natal, Republic of South Africa is reported. The complete genome sequence of JVW-186 was assembled from overlapping RT-PCR clones using MEGA 5 software. Two ORFs were identified at position 186 and 2915 of the sequence encoding the viral polyprotein and the frameshift translated protein P3N-PIPO, respectively. RDP4 software confirmed three recombination breakpoints at position 343, 1365, and 9308 of the sequence. At each recombination event, a 1,021-bp fragment at the 5' end in the region of the P1/HC-Pro protein and a 392-bp fragment in the region of the coat protein shared a high sequence similarity of 91.8 and 98.89 % to the potato borne PVY(C) isolate PRI-509 and the PVY(O) isolate SASA-110, respectively. The non-recombinant fragment 1 (342-bp) clustered within the C clade of PVY isolates; however, the large 7,942-bp fragment 3 did not cluster within any of the clades. This suggests the possibility of a PVY isolate that has evolved due to the dynamics of selection pressure or the likelihood of an ancestral PVY strain.

The influence of cis-acting P1 protein and translational elements on the expression of Potato virus Y helper-component proteinase (HCPro) in heterologous systems and its suppression of silencing activity

In the Potyvirus genus, the P1 protein is the first N-terminal product processed from the viral polyprotein, followed by the helper-component proteinase (HCPro). In silencing suppression patch assays, we found that Potato virus Y (PVY) HCPro expressed from a P1-HCPro sequence increased the accumulation of a reporter gene, whereas protein expressed from an HCPro sequence did not, even with P1 supplied in trans. This enhancing effect of P1 has been noted in other potyviruses, but has remained unexplained. We analysed the accumulation of PVY HCPro in infiltrated tissues and found that it was higher when expressed from P1-HCPro than from HCPro sequences. Co-expression of heterologous suppressors increased the steady-state level of mRNA expressed from the HCPro sequence, but not that of protein. This suggests that, in the absence of P1 upstream, either HCPro acquires a conformation that affects negatively its activity or stability, or that its translation is reduced. To test these options, we purified HCPro expressed in the presence or absence of upstream P1, and found no difference in purification pattern and final soluble state. By contrast, alteration of the Kozak context in the HCPro mRNA sequence to favour translation increased partially suppressor accumulation and activity. Furthermore, protein activity was not lower than in protein expressed from P1-HCPro sequences. Thus, a direct role for P1 on HCPro suppressor activity or stability, by influencing its conformation during translation, can be excluded. However, P1 could still have an indirect effect favouring HCPro accumulation. Our data highlight the relevance of cis-acting translational elements in the heterologous expression of HCPro.

Complementation analysis of triple gene block of Potato virus S (PVS) revealed its capability to support systemic infection and aphid transmissibility of recombinant Potato virus X

Triple gene block (TGB) sequences derived from isolates of ordinary Potato virus S (PVS-O) and Chenopodium-systemic (PVS-CS) were analyzed. Although the TGB sequences did not reveal any specific difference within the 7K protein, some specific differences within the 25K and 12K ORFs were found. In order to investigate a possible functional divergence of PVS-O and PVS-CS TGB variants, these genes were propagated in chimeric Potato virus X (PVX). Both PVS TGB variants partly complemented PVX TGB in Nicotiana benthamiana. The recombinant viruses multiplied to lower titer than the wild-type PVX in N. benthamiana showed attenuated symptoms. Whereas the recombinant PVX variants were also propagated systemically in Nicotiana glutinosa, Celosia argentea, Nicotiana occidentalis and chimeric PVX bearing TGB from PVS-O in Solanum lycopersicum, neither were propagated systemically in Chenopodium quinoa nor in Nicotiana tabacum cv. Samsun nn and the PVX-resistant Solanum tuberosum cv. Szignal. The potential for recombinant viruses to be transmitted by the aphid Myzus persicae was investigated. Aphid transmission in the recombinant virus was obtained by replacing PVX TGB by TGB from the PVS-CS isolate. These results show the potential function of Carlavirus TGB in aphid transmissibility and underlines the possible biological risks from certain recombinant virus variants.

Production of plum pox virus HC-Pro functionally active for aphid transmission in a transient-expression system

Potyviruses are non-persistently transmitted by aphid vectors with the assistance of a viral accessory factor known as helper component (HC-Pro), a multifunctional protein that is also involved in many other essential processes during the virus infection cycle. A transient Agrobacterium-mediated expression system was used to produce Plum pox virus (PPV) HC-Pro in Nicotiana benthamiana leaves from constructs that incorporated the 5' region of the genome, yielding high levels of HC-Pro in agroinfiltrated leaves. The expressed PPV HC-Pro was able to assist aphid transmission of purified virus particles in a sequential feeding assay, and to complement transmission-defective variants of the virus. Also, HC-Pro of a second potyvirus, Tobacco etch virus (TEV), was expressed and found to be functional for aphid transmission. These results show that this transient system can be useful for production of functionally active HC-Pro in potyviruses, and the possible uses of this approach to study the mechanism of transmission are discussed.

The complete nucleotide sequence of a Pakistani isolate of Watermelon mosaic virus provides further insights into the taxonomic status in the Bean common mosaic virus subgroup

Watermelon mosaic virus (WMV) is a potyvirus with a worldwide distribution, but is mostly found in temperate and Mediterranean regions. The complete nucleotide (nt) sequence of a Pakistani isolate of WMV (WMV-Pk) was determined and compared with French isolate (WMV-Fr) and other closely related potyviruses. WMV-Pk showed overall identities of 94.4% (nt) and 96% (amino acid; aa) with the WMV-Fr. However, variability was observed in the 5' UTR and P1 region. Although sequence identities over most of the genome were well above 90% at both the nt and aa levels, reaching 99.6% (aa) in the CP and 100% (aa) in the 6K1 and 6K2, thereby suggesting that WMV-Pk and WMV-Fr are identical strains, but the sequence identities in the P1 region were only 80.6% (aa) and 82.8% (nt), while that in the 5' UTR was 82%. These differences may be due to different mutation phenomena of a common ancestor virus or mutations caused by different selection pressures in two different agro-ecological zones. The sequence of WMV-Pk is very close to that of Soybean mosaic virus (SMV) over most of the genome, except for the N-terminal region, which is subject to recombination between SMV and Peanut stripe virus (PSV)/Bean common mosaic virus (BCMV), as revealed by Simplot and phylogenetic analyses of N- and C-terminal P1, HC-Pro, and 5' UTR regions of the genome.

Structural analysis of tobacco etch potyvirus HC-pro oligomers involved in aphid transmission

Oligomeric forms of the HC-Pro protein of the tobacco etch potyvirus (TEV) have been analyzed by analytical ultracentrifugation and single-particle electron microscopy combined with three-dimensional (3D) reconstruction. Highly purified HC-Pro protein was obtained from plants infected with TEV by using a modified version of the virus that incorporates a histidine tag at the HC-Pro N terminus (hisHC-Pro). The purified protein retained a high biological activity in solution when tested for aphid transmission. Sedimentation equilibrium showed that the hisHC-Pro preparations were heterogeneous in size. Sedimentation velocity confirmed the previous observation and revealed that the active protein solution contained several sedimenting species compatible with dimers, tetramers, hexamers, and octamers of the protein. Electron microscopy fields of purified protein showed particles of different sizes and shapes. The reconstructed 3D structures suggested that the observed particles could correspond to dimeric, tetrameric, and hexameric forms of the protein. A model of the interactions required for oligomerization of the HC-Pro of potyviruses is proposed.

Expression of functionally active helper component protein of Tobacco etch potyvirus in the yeast Pichia pastoris

Tobacco etch potyvirus (TEV) is transmitted by aphids in a non-persistent manner with the assistance of a virus-encoded protein known as helper component (HC-Pro). To produce a biologically active form of recombinant TEV HC-Pro protein, heterologous expression in the methylotrophic yeast Pichia pastoris was used. A cDNA encoding the TEV HC-Pro region, fused to a Saccharomyces cerevisiae alpha-mating factor secretory peptide coding region, was inserted into the P. pastoris genome using a modified version of the pPIC9 vector. The expressed TEV HC-Pro protein was obtained directly from culture medium of recombinant yeast colonies; it was able to interact with TEV particles in a protein overlay binding assay, and also to assist aphid transmission of purified TEV particles to plants using the aphid Myzus persicae as vector. Our results indicate that P. pastoris provides a rapid and low-cost heterologous expression system that can be used to obtain biologically active potyvirus HC-Pro protein for in vitro transmission assays.